CODEWITHSUNDEEP
while-looptry-catchocamlimperative-programming
bruce_ricard
bruce_ricard

Reputation: 771

return type after while in a try

Almost for the first time I'm trying to write imperative code in ocaml to try to answer a question on this website, but I'm facing a little problem.

let f() =
try
  while true do
    ()
  done
with
    _ -> 2

He doesn't like this, because he thinks that this function returns unit, as it is in the try block, but the try block returns an int. So it works if I add 3 after "done", but it's really ugly since the 3 is really never returned.

How do you do this ?

Upvotes: 3

Views: 292

Answers (2)

iguerNL
iguerNL

Reputation: 464

  • while (and for) loops in OCaml are expressions that return a result of type unit.

  • In addition, when you write (try expr1 with _ -> expr2), this is an OCaml expression of type t, if expr1 and expr2 are well typed of type t (more complicated with polymorphism)

  • But, in your example, try branch has type unit whereas with branch has type int. The OCaml compiler is not happy with that.

Upvotes: 4

Martin Jambon
Martin Jambon

Reputation: 4949

Use assert false, which always raises an exception and therefore can be used where any type is expected:

let f() =
  try
    while true do
      ()
    done;
    assert false
  with
      _ -> 2

Upvotes: 7

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